Bottom Line: There is strong evidence that eating soy regularly in childhood and adolescence (when breast tissues are forming) significantly lowers the risk of developing breast cancer later in life. It is not clear that beginning to eat soy foods in adulthood positively or negatively influences the risk of breast cancer.

By far the hottest current controversy on soy concerns its relationship to breast cancer. This remains controversial because studies have shown inconsistent results.

To begin with the least controversial area, there is strong evidence that when soy is eaten regularly by girls in childhood and adolescence, it is protective against the development of breast cancer later in life. As little as one soy food serving per day may reduce eventual incidence of breast cancer by 25-45%. These conclusions are based on four epidemiological studies.^11-14 Women who start eating soy later in life do not appear to attain anything approaching the breast cancer prevention benefit seen in women who have eaten soy their entire lives, including childhood and adolescence.

In 2006, an American Cancer Society panel headed by Lawrence Kushi, ScD, published guidelines on nutrition and physical activity for cancer prevention.¹⁵ Their conclusions on soy:

Soy-derived foods are an excellent source of protein and a good alternative to meat. Soy contains several phytochemicals, some of which have weak estrogenic activity and appear to protect against hormone-dependent cancers in animal studies. Presently, there are limited data to support a potential beneficial effect of soy supplements on reducing cancer risk.¹⁶ Furthermore, adverse effects of high doses of soy supplements on the risk of estrogen-responsive cancers, such as breast or endometrial cancer, are possible.¹⁷

The role of specific foods or nutrients in cancer prevention was also discussed by Messina and Wu in a 2009 article¹⁸ on soy and breast cancer in the American Journal of Clinical Nutrition. They noted that “identifying dietary factors that affect breast cancer risk has proven to be particularly frustrating, because there are conflicting data on the role of dietary fat, fiber, and fruit and vegetables.” The same is true for soy. The fact that intensive research in this area is ongoing makes it likely that the fog will clear in the coming years. For now, eating a varied whole foods diet and not eating vast amounts of any one food seems to be the best advice.

BREAST CANCER TREATMENT

Bottom line: There is not yet a bottom line conclusion on this subject.

Because the phytoestrogens in soy are chemically related (though not identical) to (1) true (endogenous) estrogens made by the human body; (2) true estrogens eaten in the meat and milk of hormone-supplemented animals, and; (3) true estrogens taken as medicines (hormone replacement therapy, birth control pills), controversy has surrounded the use of soy or soy supplements by women with current or past breast cancer.

There is a major unresolved theoretical question. Do soy phytoestrogens helpfully block estrogen receptor sites in women so that true estrogens are unable to dock at these sites? Or do soy phytoestrogens harmfully dock at estrogen receptor sites and mimic the effects of actual estrogens, thus aggravating hormone-sensitive cancers such as breast cancer? If you are a past or present breast cancer patient, or a doctor treating such a patient, this is an extremely significant issue.

A 2006 American Cancer Society report¹⁹ on nutrition and physical activity during and after cancer treatment, published by a team led by Colleen Doyle, MS, RD, director of nutrition and physical activity at the American Cancer Society, addressed the soy issue directly: